The complement system provides an early acting mechanism to initiate and amplify the inflammatory response to microbial infection and other acute insults. (Liszewski, M. K. and J. P. Atkinson, 1993, In Fundamental Immunology, Third Edition. Edited by W. E. Paul. Raven Press, Ltd. New York). While complement activation provides a valuable first-line defense against potential pathogens, the activities of complement that promote a protective inflammatory response can also represent a potential threat to the host. (Kalli, K. R., P. Hsu, and D. T. Fearon, 1994, Springer Semin Immunopathol. 15:417-431; Morgan, B. P., Eur. J. Clinical Investig. 24:219-228). For example, C3 and C5 proteolytic products recruit and activate neutrophils. These activated cells are indiscriminate in their release of destructive enzymes and may cause organ damage. In addition, complement activation may cause the deposition of lytic complement components on nearby host cells as well as on microbial targets, resulting in host cell lysis. The growing recognition of the importance of complement-mediated tissue injury in a variety of disease states underscores the need for effective complement inhibitory drugs. No approved drugs that inhibit complement damage currently exist.
Complement can be activated through either of two distinct enzymatic cascades, referred to as the classical and alternative pathways. (Liszewski, M. K. and J. P. Atkinson, 1993, In Fundamental Immunology, Third Edition. Edited by W. E. Paul. Raven Press, Ltd. New York). The classical pathway is usually triggered by antibody bound to a foreign particle and thus requires prior exposure to that particle for the generation of specific antibody. There are four plasma proteins specifically involved in the classical pathway: C1, C2, C4 and C3. The interaction of C1 with the Fc regions of IgG or IgM in immune complexes activates a C1 protease that can cleave plasma protein C4, resulting in the C4a and C4b fragments. C4b can bind another plasma protein, C2. The resulting species, C4b2, is cleaved by the C1 protease to form the classical pathway C3 convertase, C4b2a. Addition of the C3 cleavage product, C3b, to C3 convertase leads to the formation of the classical pathway C5 convertase, C4b2a3b.
In contrast to the classical pathway, the alternative pathway is spontaneously triggered by foreign or other abnormal surfaces (bacteria, yeast, virally infected cells, or damaged tissue) and is therefore capable of an immediate response to an invading organism (Liszewski, M. K. and J. P. Atkinson, 1993, In Fundamental Immunology, Third Edition. Edited by W. E. Paul. Raven Press, Ltd. New York). There are four plasma proteins directly involved in the alternative pathway: C3, factors B and D, and properdin (also called factor P). The initial interaction that triggers the alternative pathway is not completely understood. However, it is thought that spontaneously activated C3 [sometimes called C3(H.sub.2 O)] binds factor B, which is then cleaved by factor D to form a complex [C3(H.sub.2 O)Bb] with C3 convertase activity. The resulting convertase proteolytically modifies C3, producing the C3b fragment, which can covalently attach to the target and then interact with factors B and D and form the alternative pathway C3 convertase, C3bBb. The alternative pathway C3 convertase is stabilized by the binding of properdin. Properdin extends its half-life six-to ten-fold (Liszewski, M. K. and J. P. Atkinson, 1993, In Fundamental Immunology, Third Edition. Edited by W. E. Paul. Raven Press, Ltd. New York). However, properdin binding is not required to form a functioning alternative pathway C3 convertase (Schreiber, R. D., M. K. Pangburn, P. H. Lesavre and H. J. Muller-Eberhard, 1978, Proc. Natl. Acad. Sci. USA 75:3948-3952; Sissons, J. G., M. B. Oldstone and R. D. Schreiber, 1980, Proc. Natl. Acad. Sci. USA 77:559-562). Since the substrate for the alternative pathway C3 convertase is C3, C3 is therefore both a component and a product of the reaction. As the C3 convertase generates increasing amounts of C3b, an amplification loop is established (Liszewski, M. K. and J. P. Atkinson, 1993, In Fundamental Immunology, Third Edition. Edited by W. E. Paul. Raven Press, Ltd. New York). Inasmuch as the classical pathway also may generate C3b, that C3b can bind factor B and thereby engage the alternative pathway. This allows more C3b to deposit on a target. For example, as described above, the binding of antibody to antigen initiates the classical pathway. If antibodies latch on to bacteria, the classical pathway generates C3b, which couples to target pathogens. However, it has been suggested that from 10% to 90% of the subsequent C3b deposited may come from the alternative pathway (Liszewski, M. K. and J. P. Atkinson, 1993, In Fundamental Immunology, Third Edition. Edited by W. E. Paul. Raven Press, Ltd. New York). The actual contribution of the alternative pathway to the formation of additional C3b subsequent to classical pathway initiation has not been clearly quantified and thus remains unknown. Addition of C3b to the C3 convertase leads to the formation of the alternative pathway C5 convertase, C3bBbC3b.
Both the classical and alternative pathways converge at C5, which is cleaved to form products with multiple proinflammatory effects. The converged pathway has been referred to as the terminal complement pathway. C5 a is the most potent anaphylatoxin, inducing alterations in smooth muscle and vascular tone, as well as vascular permeability. It is also a powerful chemotaxin and activator of both neutrophils and monocytes. C5a-mediated cellular activation can significantly amplify inflammatory responses by inducing the release of multiple additional inflammatory mediators, including cytokines, hydrolytic enzymes, arachidonic acid metabolites and reactive oxygen species. C5 cleavage leads to the formation of C5b-9, also known as the membrane attack complex (MAC). There is now strong evidence that MAC may play an important role in inflammation in addition to its role as a lytic pore-forming complex (Liszewski, M. K. and J. P. Atkinson, 1993, In Fundamental Immunology, Third Edition. Edited by W. E. Paul. Raven Press, Ltd. New York).
Complement activation has been implicated as contributing to a variety of disease states and conditions, as well as complications from a variety of medical procedures (see references cited infra) such as: myocardial infarction; ischemia/reperfusion injury; stroke; acute respiratory distress syndrome (ARDS); sepsis; burn injury; complications resulting from extracorporeal circulation (ECC) including most commonly from cardiopulmonary bypass (CPB) but also from hemodialysis or plasmapheresis or plateletpheresis or leukophereses or extracorporeal membrane oxygenation (ECMO) or heparin-induced extracorporeal LDL precipitation (HELP); use of radiographic contrast media; transplant rejection; rheumatoid arthritis; multiple sclerosis; myasthenia gravis; pancreatitis; and Alzheimer's disease. There is still no effective complement inhibitory drug available for routine clinical use despite the significant medical need for such agents.
The ability to specifically inhibit only the pathway causing a particular pathology without completely shutting down the immune defense capabilities of complement would be highly desirable. Based upon the available clinical data, it appears that in most acute injury settings, complement activation is mediated predominantly by the alternative pathway (Moore, F. D. 1994, Advan. Immunol. 56:267-299; Bjornson, A. B., S. Bjornson and W. A. Altemeier, 1981 Ann. Surg. 194:224-231: Gelfand, J. A., M. Donelan, and J. F. Burke, 1983, Ann. Surg. 198:58-62). These findings suggest that it would be advantageous to specifically inhibit alternative pathway-mediated tissue damage in a variety of acute injury settings, for example, in myocardial infarction, ARDS, reperfusion injury, stroke, thermal burns, and post-cardiopulmonary bypass inflammation. This would leave the classical pathway intact to handle immune complex processing and to aid in host defense against infection. As essential components of the alternative pathway, factors B and D are attractive targets for specific inhibition of the alternative pathway. Because of its non-essential role, properdin, however, would not be expected to be a suitable target for such intervention.